This invention relates to a direct-radiation slotted rectangular waveguide having a wide frequency band.
In the field of radar antennas, a particularly simple and compact antenna consists of a rectangular waveguide having radiating slots excited by traveling waves, the operation of which will now be recalled.
In the first place, a slot radiates power when it intersects current lines. Since it can in fact be compared with an impedance Z placed in series on the current lines, a potential difference appears between the walls of the slot and this consequently produces radiation to the exterior.
In accordance with Babinet's principle, it is deduced in the second place that the field radiated by a slot has the same nature at that which is radiated by a dipole having the same width, their respective polarizations being perpendicular.
Furthermore, since the power radiated by the slot is proportional to the square of the current which flows through said slot, coupling of the slot with the waveguide can accordingly be adjusted by choosing its position and its angle of inclination.
In accordance with conventional practice and as shown in FIG. 1a, the slots 1 can be placed longitudinally along the broad side 2 of the waveguide 3 and displaced off-center to a greater or lesser extent or else placed transversely on the narrow side 4 of the waveguide in more or less inclined positions as shown in FIG. 1b. Although they offer the advantage of radiating practically the entire waveguide power, said slots suffer from a disadvantage in that they have conductances which vary rapidly as a function of the frequency, thereby producing a variation in coupling of the slots with the waveguide and instability of the law of illumination which governs the radiation pattern and particularly the sidelobes.
A complex solution has been found in answer to this problem by exciting each radiating slot of the waveguide by means of a directional coupler extending within the guide but the construction involved is complex.
The object of the present invention is to provide a direct-radiation slotted rectangular waveguide which offers the further advantage of operating over a wide frequency band.
The direct-radiation slotted rectangular waveguide according to the invention is such that each radiating slot having a length L in the vicinity of the operating wavelength (.lambda.) of the waveguide is placed on one side of the waveguide so as to be parallel to the lines of current flow along said side and is provided with a transverse stepped section formed in its central portion at right angles to the current lines.
According to one distinctive feature of the invention, the slots are formed on either a broad or a narrow side of the waveguide.
Other features of the invention will be more apparent upon consideration of the following description, reference being made to FIGS. 2 to 5 of the accompanying drawings which, apart from FIG. 1 which relates to the prior art, illustrate examples of construction of a radiating-slot waveguide according to the invention.